1. Field of the Invention
The present invention relates to an engine configured to drive a propulsion mechanism of a watercraft, and a personal watercraft comprising the engine as a drive source of the propulsion mechanism.
2. Description of the Related Art
In recent years, jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. A typical personal watercraft includes an engine mounted in a space within the watercraft that is surrounded by a hull and a deck. The engine is configured to drive a water jet pump, which pressurizes and accelerates water sucked from a water intake generally provided on a hull bottom surface, and ejects it rearward from an outlet port. As the resulting reaction, the personal watercraft is propelled forward.
In a personal watercraft, typically, water (e.g., sea water or lake water) that has been pressurized by the water jet pump is partially drawn from an inside of the water jet pump through a water-drawing hole provided in a pump casing, for use as cooling water to cool various engine components, such as a cylinder head and a cylinder block, as well as auxiliary equipment such as an exhaust device. Such a cooling system is called an open-looped cooling system (or direct cooling system), and is disclosed in Published Unexamined Japanese Utility Model No. H02-100896.
Furthermore, the cylinder head, the exhaust device and the like tend to heat up to relatively high temperatures. In order for these components to be sufficiently cooled, prior open-looped cooling systems include a pump casing with a water-drawing hole having a relatively large diameter, to enable a large amount of water to be drawn up through the water-drawing hole to cool these components. Typically, the cooling water is required to be drawn from a location within the water jet pump where static pressure of the water is stable. Since such a location lies within a relatively narrow region (hereinafter, referred to as a “stable static-pressure region”), large-diameter water-drawing holes of prior open-looped cooling systems tend to extend at least partially outside the stable static-pressure region. This is problematic, because it reduces water-drawing efficiency, making it difficult to take in a large amount of cooling water.
The reduced water-drawing efficiency occurs because the water jet pump is driven in cooperation with rotation of the engine, and the pressure of the water flowing within the pump varies with an engine speed of the engine. Thus, when the water-drawing hole partially extends outside the stable static-pressure region, the cooling water being drawn up significantly decreases with decreasing engine speed.
In contrast to the above described direct cooling system, in prior indirect cooling systems, coolant circulates within an engine while cooling components that generate heat, such as a cylinder block. Heat is exchanged between the coolant and air or water taken in from outside to allow the heat to be released to outside the watercraft. These prior indirect cooling systems must have a cooling water passage with a relatively large heat-release area, for the components to be cooled appropriately. For example, one prior indirect cooling system includes a water jacket provided within the cylinder block designed so that a dimension in a piston stroke direction (piston reciprocation direction) is maximized for the purpose of a larger heat-release area of the cooling water passage.
The cooling water used in prior direct cooling systems employed in personal watercraft is generally lake or sea water, which typically has a temperature lower than that of the coolant used in indirect cooling systems. When a direct cooling system is applied to an engine that also is equipped with an indirect cooling system, the cylinder block is undesirably cooled excessively, thereby causing a friction loss due to friction between the cylinder block and a piston which reciprocates within the cylinder block to increase. By drawing up the water in limited amount, the excess cooling, and hence the increase in the friction loss can be inhibited, but the cylinder head and the exhaust device whose temperatures become high tend to be insufficiently cooled.
The friction loss caused by the excess cooling of the cylinder block occurs in both two-cycle engines and four-cycle engines. In addition to the friction loss, in four-cycle engines, excess cooling causes fuel to be insufficiently vaporized within the combustion chambers. Uncombusted fuel may be undesirably mixed with lubricating oil in the engine, which causes dilution of the oil.